1. Field of the Invention
The present invention relates to an absorbent composition and to a process for the selective absorption of H2S from an H2S and other acidic components containing mixture using the absorbent composition.
2. Description of the Related Art
It is well known in the art to treat gases and liquids, such as mixtures containing acidic gases including CO2, H2S, CS2, HCN, COS and oxygen and sulfur derivatives of C1 to C4 hydrocarbons with amine solutions to remove these acidic gases. The amine usually contacts the acidic gases and the liquids as an aqueous solution containing the amine in an absorber tower with the aqueous amine solution contacting the acidic fluid countercurrently.
The treatment of acid gas mixtures containing, inter alia, CO2 and H2S with amine solutions typically results in the simultaneous removal of substantial amounts of both the CO2 and H2S. For example, in one such process generally referred to as the “aqueous amine process”, relatively concentrated amine solutions are employed. A recent improvement of this process involves the use of sterically hindered amines as described in U.S. Pat. No. 4,112,052, to obtain nearly complete removal of acid gases such as CO2 and H2S. This type of process may be used where the partial pressures of the CO2 and related gases are low. Another process often used for specialized applications where the partial pressure of CO2 is extremely high and/or where many acid gases are present, e.g., H2S, COS, CH3SH and CS2 involves the use of an amine in combination with a physical absorbent, generally referred to as the “nonaqueous solvent process”. An improvement on this process involves the use of sterically hindered amines and organic solvents as the physical absorbent such as described in U.S. Pat. No. 4,112,051.
It is often desirable, however, to treat acid gas mixtures containing both CO2 and H2S so as to remove the H2S selectively from the mixture, thereby minimizing removal of the CO2. Selective removal of H2S results in a relatively high H2S/CO2 ratio in the separated acid gas which simplifies the conversion of H2S to elemental sulfur using the Claus process.
The typical reactions of aqueous secondary and tertiary amines with CO2 and H2S can be represented as follows:H2S+R3NR3NH++SH−  (1)H2S+R2NHR2NH2++SH−  (2)CO2+R3N+H2OR3NH++HCO3−  (3)CO2+2R2NHR2NH2++R2NCOO−  (4)RNH2+CO2RN+H2CO2−  (5)RN+H2CO2+RNH2RNHCO2−RNH3+  (6)wherein each R is an organic radical which may be the same or different and may be substituted with an hydroxy group. The above reactions are reversible, and the partial pressures of both CO2 and H2S are thus important in determining the degree to which the above reactions occur.
While selective H2S removal is applicable to a number of gas treating operations including treatment of hydrocarbon gases from shale pyrolysis, refinery gas and natural gas having a low H2S/CO2 ratio, it is particularly desirable in the treatment of gases wherein the partial pressure of H2S is relatively low compared to that of CO2 because the capacity of an amine to absorb H2S from the latter type gases is very low. Examples of gases with relatively low partial pressures of H2S include synthetic gases made by coal gasification, sulfur plant tail gas and low-Joule fuel gases encountered in refineries where heavy residual oil is being thermally converted to lower molecular weight liquids and gases.
Although it is known that solutions of primary and secondary amines such as monoethanolamine (MEA), diethanolamine (DEA), dipropanolamine (DPA), and hydroxyethoxyethylamine (DGA) absorb both H2S and CO2 gas, they have not proven especially satisfactory for preferential absorption of H2S to the exclusion of CO2 because the amines undergo a facile reaction with CO2 to form carbamates as shown in Equations 5 and 6.
Diisopropanolamine (DIPA) is relatively unique among secondary amino alcohols in that it has been used industrially, alone or with a physical solvent such as sulfolane, for selective removal of H2S from gases containing H2S and CO2, but contact times must be kept relatively short to take advantage of the faster reaction of H2S with the amine compared to the rate of CO2 reaction shown in Equations 2 and 4 hereinabove.
In 1950, Frazier and Kohl, Ind. and Eng. Chem., 42, 2288 (1950) showed that the tertiary amine, methyldiethanolamine (MDEA), has a high degree of selectivity toward H2S absorption over CO2. This greater selectivity was attributed to the relatively slow chemical reaction of CO2 with tertiary amines as compared to the rapid chemical reaction of H2S. The commercial usefulness of MDEA, however, is limited because of its restricted capacity for H2S loading and its limited ability to reduce the H2S content to the level at low pressures which is necessary for treating, for example, synthetic gases made by coal gasification.
Recently, U.K. Patent Publication No. 2,017,524A to Shell disclosed that aqueous solutions of dialkylmonoalkanolamines, and particularly diethyl-monoethanolamine (DEAE), have higher selectivity and capacity for H2S removal at higher loading levels than MDEA solutions. Nevertheless, even DEAE is not very effective for the low H2S loading frequency encountered in the industry. Also, DEAE has a boiling point of 161° C., and as such, it is characterized as being a low-boiling, relatively highly volatile amino alcohol. Such high volatilities under most gas scrubbing conditions result in large material losses with consequent losses in economic advantages.
U.S. Pat. Nos. 4,405,581; 4,405,583 and 4,405,585 disclose the use of severely sterically hindered amine compounds for the selective removal of H2S in the presence of CO2. Compared to aqueous methyldiethanolamine (MDEA) severely sterically hindered amines lead to much higher selectivity at high H2S loadings.
U.S. Pat. No. 4,112,052 is directed to a process for removing CO2 from acid gases using an aqueous amine scrubbing solution. The amines used are sterically hindered amines containing at least one secondary amine group attached to either a secondary or tertiary carbon atom or a primary amino group attached to a tertiary carbon atom. The amines are selected to be at least partially soluble in the solvent used, i.e., water.
U.S. Pat. No. 4,376,102 discloses that acidic gases containing CO2 are removed from normally gaseous mixtures by absorbing the CO2 from the gaseous mixture using an aqueous solution comprising a basic alkali metal salt or hydroxide which contains (1) at least one diaminoalcohol of the formula
wherein R and R1 are each independently a C1-C6 alkyl group and either R or R1 or both R and R1 have a pendent hydroxyl group and (2) an amino acid. The basic alkali metal salt or hydroxide are selected from the group consisting of alkali metal bicarbonates, carbonates, hydroxides, borates, phosphates and their mixtures. See also U.S. Pat. Nos. 4,376,101; 4,581,209; 4,217,238.
U.S. Pat. No. 4,525,294 is directed to amino acid mixtures, their alkali metal salts and processes for their preparation. The process involves the reductive condensation of glycine or alanine and their alkali metal salts with a ketone in the presence of a reductant such as hydrogen and a catalytically effective amount of an hydrogenation catalyst. Thus, a reaction as follows is disclosed:
wherein R is hydrogen or methyl, X is hydrogen or an alkali metal such as sodium or potassium, R′ and R″ are selected from the group consisting of:    a) substituted or unsubstituted linear or branched allyl radicals having one to 20 carbons; or    b) substituted or unsubstituted alkylene radicals each having three to six carbon atoms and combined to form a cyclic ring;    c) substituted or unsubstituted cycloalkyl radicals having from four to eight ring carbon atoms;    d) substituted or unsubstituted hydroxyl alkyl radicals, linear or branched, having one to 20 carbon atoms; or    e) substituted or unsubstituted arylalkyl radicals having from seven to 20 carbon atoms;and R′″ is hydrogen or a substituted or unsubstituted linear alkyl radical having from 1 to 20 carbon atoms, or mixtures of hydrogen and such alkyl radicals.
U.S. Pat. No. 4,759,866 discloses primary sterically hindered amino acids of the formula:
wherein R1 and R2 are independently selected from CH3, C2H5 and C3H7, and R3 and R4 are independently hydrogen and CH3 and n is zero, 2 or 3, for use as promoters for alkali metal salts in acid gas scrubbing.
U.S. Pat. No. 5,602,279 is directed to a gas treating composition prepared by reacting 2-amino-2-methyl-1-propanol with KOH, diluting with water and adding K2CO3 and a vanadium corrosion inhibitor. The acid gas scrubbing solution contains

U.S. Pat. No. 4,618,481 is directed to an absorbent composition comprising a severely hindered amino compound and an amine salt for the absorption of H2S from gaseous mixtures. The severely sterically hindered amino compound can be a secondary amino ether alcohol, a disecondary amino ether, and mixtures thereof. The amine salt can be the reaction product of the aforesaid severely sterically hindered amino compound, a tertiary amino compound such as tertiary alkanolamines, triethanol amines, and mixtures thereof and a strong acid, or a thermally decomposable salt of a strong acid, i.e., ammonium salt or a component capable of forming a strong acid and mixtures thereof. Suitable strong acids include inorganic acids such as sulfuric acid, sulfurous acid, phosphoric acid, phosphorous acid, pyrophosphoric acid, an organic acid such as acetic acid, formic acid, adipic acid, benzoic acid, etc. Suitable salts of these acids include the ammonium salts, for example ammonium sulfate, ammonium sulfite, ammonium phosphate and mixtures thereof. Preferably ammonium sulfate (a salt) or SO2 (a precursor of an acid) is used as reactant with the amine. Suitable amine salts are those that are non-volatile at conditions used to regenerate the absorbent composition.
U.S. Pat. No. 4,892,674 is directed to an absorbent composition comprising an alkaline absorbent solution containing a non-hindered amine and an additive of a severely-hindered amine salt and/or a severely-hindered aminoacid and to the use of the absorbent for the selective removal of H2S from gaseous streams. The amine salt is the reaction product of an alkaline severely hindered amino compound and a strong acid or a thermally decomposable salt of a strong acid, i.e., ammonium salt. Suitable strong acids include inorganic acids such as sulfuric acid, sulfurous acid, phosphoric acid, phosphorous acid, pyrophosphoric acid; organic acids such as acetic acid, formic acid, adipic acid, benzoic acid, etc. Suitable salts include the ammonium salts, for example, ammonium sulfate, ammonium sulfite, ammonium phosphate and mixtures thereof.